In order treat bone fractures, in particular fractures in long bones such as the tibia or the femur, it is common practice to use the technique of centro-medullary nailing. This technique consists in inserting a metal rod in a prior-formed bore in the centro-medullary channel of the fractured bone, which metal rod begins by serving to reduce the fracture and then serves to maintain the disjoint parts of the bone in the correct positions during the subsequent stage of osteogenesis leading to said parts knitting together.
The rods used for this purpose are sometimes called "Kuntscher nails".
Such a rod is a metal tube which is generally made of stainless steel and which is split longitudinally. It thus has a C-shaped section. The slot which is several millimeters wide gives the nail a degree of resilience in the radial direction, since the edges of the slot can move towards each other or away from each other resiliently. This resilience makes it possible to obtain intimate contact between the outer wall of the nail and the bore wall of the bone, which is necessary in order to hold the disjoint parts properly.
The presence of this slot has practically no effect on the bending strength of the rod. However, the slot very considerably reduces the twisting strength of the rod. If a tube is split longitudinally, it deforms much more easily under the effect of a twisting couple than it would if it had not been split, with the two edges of the slot tending to move towards each other while describing a helix.
It will readily be understood that excessive flexibility in twisting is undesirable since it allows relative displacement to occur between the disjoint parts, thereby hindering and slowing down consolidation of bone tissue around the fracture.
That is why, in order to obtain adequate twisting strength, manufacturers have been obliged, heretofore, to provide for this type of nail to have a relatively thick wall, e.g. about 2 mm or more, which would otherwise not be justified. Unfortunately, excessive wall thickness correspondingly reduces the radial resilience of the nail, and the contradictory requirements relating to twisting stiffness and to radial resilience give rise to a compromise which is not always entirely satisfactory. Further, large wall thickness leads to high weight, which constitutes a drawback for the patient since this type of rod must be retained in the bone over long periods of time.
Further, the document EP-A-0 145 666 describes an intramedullary nail which includes a longitudinal slot extending over the entire length of the nail. This characteristic is intended to avoid the stress concentrations which occur at the end zone of the slot in conventional nails having an incomplete slot, for the purpose of ensuring that the nail is of uniform resilience over its entire length. However, in order to avoid difficulties when the nail is being put into place or is being removed, the width of the slot in the internally-tapped proximal portion of the nail is small compared with a conventional slot. In this portion, an intermeshing dove-tailed shape is provided for the purpose of keeping the two edges of the slot close together and preventing this portion of the nail from expanding.
This prior nail which includes a conventional slot over the major portion of its length has twisting strength which is no better than that of conventional nails.
The invention seeks to solve these various problems by providing a centro-medullary nailing rod of the type mentioned above which, while retaining good resilience in the radial direction, possesses increased strength in twisting.